This invention relates generally to the field of optical isolators and more specifically to an optical isolator for use in high power applications.
Optical isolators are one of the most ubiquitous of all passive optical components found in optical communication systems. Generally, optical isolators are used to allow signals to propagate in a forward direction but not in a backward direction. They are frequently used to prevent unwanted back reflections from being transmitted back to a transmitting source such as a laser.
Referring to FIG. 1a, there is shown a polarization insensitive optical isolator. The isolator 2 includes an isolator core 4 comprising a first birefringent crystal 8, a non-reciprocal rotator in the form of a Faraday rotator 10, a reciprocal rotator in the form of a half-waveplate 12, and a second birefringent crystal 14. The Faraday rotator 10 is typically formed from doped garnet or YIG, and is placed in a permanent magnet. On one side of the isolator core is placed an input optical fiber 18 and on the other side is placed an output optical fiber 20. The jacket 18c of input optical fibre 18 is stripped away and an exposed end of the core 18a and cladding 18b of the input 18 optical fibre is secured in a ferrule 26 with epoxy 30. Similarly, an exposed end of the core 20a and cladding 20b of the output 20 fibre is secured in a ferrule 28 with epoxy 31. Preferably, the ferrules 26 and 28 have a predetermined angle selected to reduce backreflection. Lens 40 and optional spacer 42 are optically coupled to the ferrule 26 and optical fibre 18, and held in place with a sleeve 44. Similarly, lens 50 and optional spacer 52 are optically coupled to ferrule 28 and optical fibre 20, and are held in place with sleeve 54.
In operation, an optical signal launched from the core 18a of the input fibre is collimated and transmitted through the first birefringent crystal 8, where it is separated into two orthogonally polarized sub-beams of light. More specifically, the two orthogonal sub-beams are depicted as the O-ray and the E-ray components, wherein the E-ray experiences a spatial displacement as it traverses the birefringent crystal 8. The two rays pass through the Faraday rotator 10 wherein the polarization of each sub-beam is rotated by 45xc2x0 and the half waveplate 12 wherein the polarization of each sub-beam is rotated another 45xc2x0, for a total rotation of about 90xc2x0. Since the polarization of each ray is rotated by 90xc2x0, the E-ray component is unaffected as it passes through the second birefringent crystal 14, whereas the O-ray component experiences spatial displacement. The two rays are recombined and focussed onto the core of the output fibre 20a. 
Referring to FIG. 1b, a ray diagram showing an optical signal launched from the core of the output fibre 20a to the second birefringent crystal 14 is shown. The second birefringent crystal separates the optical signal into two orthogonal rays corresponding to the O-ray and the E-ray components, wherein the E-ray experiences a spatial displacement as it traverses the birefringent crystal 8. The two rays of light are then passed through the half waveplate 12 and the Faraday rotator 10. Since the half waveplate 12 is a reciprocal device, whereas the Faraday rotator 10 is a non-reciprocal device, a total rotation of about 0xc2x0 is observed and the first birefringent crystal 8 does not recombine the two rays. More specifically, the E-ray component experiences a further spatial displacement, whereas the O-ray component passes through the second birefringent crystal 14 unaffected, such that the two rays are focussed on points away from the core 18a of the first optical fibre, thus providing isolation in the reverse direction.
Improvements, or modifications, in optical isolators include adding a reflector, replacing the half waveplate with a third birefringent crystal, designing the optical components with wedge angles, adding lenses, adding polarization diversity, and/or adding additional components for improving isolation (e.g., multi-stage optical isolators). For example, see U.S. Pat. Nos. 5,033,830, 5,208,876, 5,345,329, 5,734,762, and 6,088,153 incorporated herein by reference.
As described above, a disadvantage of the isolator shown in FIGS. 1a and 1b is that it directs the backward propagating light to points surrounding the input core 18a. For example, the backward propagating light is directed to locations ranging from the input optical fibre cladding 18b to the ferrule 26, depending on the length of the first birefringent crystal (e.g., 8) and the optical arrangement. For low power applications this may be acceptable, however, for high power applications this results in significant damage of the optics and/or system degradation/failure. For example, one significant problem arises when backward propagating high power light is transmitted to the ferrule and degrades the epoxy (e.g., 30) used to secure and align the optical components. The degradation of epoxy in optical isolators is discussed in U.S. Pat. No. 5,661,829, incorporated herein by reference. Other examples include the burning or damaging of other optics due to excessive heating and/or high power radiation.
In U.S. Pat. No. 5,546,486 to Shih et al. there is disclosed an optical isolator including an input fiber having a reflective optical barrier layer, such as gold, that covers the end surface of the fiber with an aperture exposing the core and covering the cladding of the fiber. Although, this improvement substantially reduces light transmission into the end of the optical fiber via the cladding, thus improving the isolation, it does not protect the other optical components from the high power radiation. In fact, the device taught by Shih et al. is not compatible with high power applications, since the reflective layer produces additional reflections that may introduce noise and/or damage other optical components. For example, if the reflective layer is soiled with dust or another impurity, it might ignite in high power applications.
It is an object of this invention to provide an optical isolator that obviates the above mentioned disadvantages.
It is a further object of this invention to provide an optical isolator for use in high power applications.
The instant invention provides an optical isolator for use in high power applications that includes a light collector for redirecting and/or absorbing backward propagating radiation. In the preferred embodiment, the light collector does not interfere with forward propagating radiation, but collects or gathers the backward propagating light.
In accordance with the invention there is provided a method for protecting isolator components from high intensity backreflections in an optical isolator comprising a first port for launching light in a forward propagating direction, a second port for receiving the light launched from the first port and for transmitting light in a backward propagating direction towards the first port, and an isolator core optically disposed between the first and second ports including a first birefringent crystal, a non-reciprocal rotator, and a second birefringent crystal, the method comprising the step of: providing light collecting means for substantially unaffecting forward propagating light launched from the first port and for collecting and isolating backward propagating light transmitted from the isolator core.
In accordance with the invention there is provided an optical isolator comprising: a first port; a second port optically coupled to the first port; an isolator core optically disposed between the first port and the second port comprising a first birefringent crystal, a non-reciprocal rotator, and a second birefringent crystal for transmitting forward propagating light from the first port to the second port and for preventing backward propagating light transmitted from the second port from coupling to the first port; and light collecting means for substantially collecting backward propagating light transmitted from the isolator core to prevent damage to other optical components without substantially affecting forward propagating light launched from the first port.
In accordance with the invention there is provided an optical isolator comprising: a first port; a second port optically coupled to the first port; an isolator core comprising a first birefringent crystal, a non-reciprocal rotator, a reciprocal rotator, and a second birefringent crystal disposed such that a forward propagating beam of light launched from the first port is separated into two sub-beams of light having orthogonal polarizations by the first birefringent crystal, which are recombined into a single beam of light by the second birefringent crystal and transmitted to the second port, and such that a backward propagating beam of light transmitted from the second port is separated into two backward propagating sub-beams of light having orthogonal polarizations by the second birefringent crystal, which are further spatially separated by the first birefringent crystal and transmitted away from the first port; and light collecting means disposed for substantially collecting the two backward propagating further spatially separated sub-beams of light to prevent damage to other optical components.
In accordance with the invention there is provided an optical isolator comprising: a first optical fibre having an exposed end including a core and a cladding secured in a first ferrule; a second optical fibre optically coupled to the first optical fibre having an exposed end including a core and a cladding secured in a second ferrule; an isolator core optically disposed between the first and second optical fibres comprising a first birefringent crystal, a non-reciprocal rotator, a reciprocal rotator, and a second birefringent crystal for directing forward propagating light from the core of the first optical fibre to the core of the second optical fibre and for directing backward propagating light transmitted from the core of the second optical fibre away from the core of the first optical fibre; and light collecting means for collecting backward propagating light transmitted from the isolator core and substantially preventing it from impinging on the first ferrule without substantially affecting forward propagating light.